1. Field of the Invention
The present invention relates to a radiographic image capturing apparatus and a radiographic image capturing method for performing radiation image capturing.
2. Description of the Related Art
In recent years, medical radiographic image capturing apparatuses have generated digital images instead of analog images for use in diagnosing. Such a medical radiographic image capturing apparatus continuously captures radiographic digital images, and displays them on a monitor as a moving image or stores them in a memory or a hard disk. These captured radiographic digital images are used for diagnosing. A user of such a medical radiographic image capturing apparatus can obtain a still image useful for a diagnostic workup in such a manner that the user increases the dose of radiation in a desired scene while observing a moving image displayed on a monitor. The captured still image is stored in a memory or a hard disk.
In radiographic image capturing apparatuses, it is generally required to optimally set image capturing parameters (a radiation control variable, a device control variable, an image processing variable, etc.) so that still images useful for diagnosing can be obtained. An empirical value is usually derived on the basis of information about a part of a subject and is then set as an image capturing parameter. In some cases, an image pre-captured prior to main image capturing is analyzed, and an image capturing parameter is calculated and set on the basis of the analysis result.
On the other hand, in a case in which the image of a periodically moving subject such as lungs or a heart is captured, it is required to capture the image thereof at an optimum time for diagnosing. For example, the image of a human lung field can be captured at a desired time and in a desired condition (breath holding at the end of inhalation) under instructions from a radiographer or an operator. However, if the image of a heart, an unconscious patient (person), an animal, or a non-living material such as a machine is captured, it is difficult to perform image capturing at an optimum time. For example, Japanese Patent Laid-Open No. 2003-290184 discloses a technique used for human chest diagnosing in which a lung inflation cycle is detected and the main image capturing of a still image of lungs is performed on the basis of the detection result.
In the above-described image capturing, the reduction in the dose of radiation applied to a subject is needed. More specifically, Japanese Patent Laid-Open No. 5-192319 discloses a technique for obtaining a diagnostic image with small doses of radiation by detecting the change in a subject using captured images to be used for diagnosing and changing a frame rate or the dose of radiation on the basis of the detection result.
However, since experience is required to optimally set an image capturing parameter used for the main image capturing of a still image, it is difficult for unskilled users to obtain an image useful for diagnosing.
If the above-described method of analyzing pre-captured images and setting an effective radiation control parameter on the basis of the analysis result is used, it is required to use images pre-captured with the same irradiation dose as for the main image capturing or an irradiation dose that is lower than that for the main image capturing but is still relatively high. Accordingly, the reduction in the dose of radiation applied to a subject cannot be achieved.